Modern paper manufacturing involves a complex series of mechanical and chemical processes that converts wood pulp into a finished paper product. During the manufacturing process, papermaking fibers formed from wood pulp are distributed along a moving wire screen and eventually are formed and pressed into a continuous sheet of fibers referred to as a “paper web.” The paper web is further processed by elaborate drying and pressing processes to reduce the moisture in the web and form bonds between the fibers. Once dried, various surface treatments may be applied to the web, depending on the application for which the paper is ultimately used. This complex procedure requires precision control and oversight to create high quality paper products. Accordingly, paper manufacturers are constantly looking for new and improved methods of measuring and assessing the characteristics of the paper formed in their manufacturing facilities.
One important characteristic of paper is how flat its surface is after manufacture. Defects resulting from various manufacturing processes create raised or depressed regions that diverge from the plane parallel to the paper surface. These defects are referred to as “out-of-plane defects” and have a height that deviates from the average levelness of a flat sheet. There are a number of out-of-plane defects recognized by the paper industry. For instance, a “streak” appears as a sinuous line of variable length that typically appears on paper in the direction of the paper web's movement (i.e., the machine direction). Although streaks may be straight, they often wander to some degree in the direction transverse to the direction of the paper web (i.e., the cross direction). “Puckers,” appear as small semi-circular raised areas on the surface of the paper, whereas “cockles” appear as waves in the plane of the paper surface.
Several techniques have been developed to analyze various paper characteristics. Few are capable of measuring out-of-plane defects and no known technique allows for an entire paper sample to be evaluated quickly and easily.
For instance, U.S. Pat. No. 5,243,407 to King et al. discloses a device for characterizing the formation of a sheet of paper, where “formation” is defined as “the manner in which fibers forming a paper sheet are distributed, disposed and intermixed within the sheet.” Col. 1, lns. 48-41. The device includes a basis weight sensor “for accurately measuring local variations in the basis weight of a sheet of paper,” where the sensor includes a light beam source “disposed on one side of the sheet” and a receiver “disposed on the other side of the sheet opposing the light beam source.” Col. 3, lns. 48-52.
U.S. Pat. No. 5,899,959 to Shields et al. discloses an apparatus for determining visual characteristics of a paper web including “formation, moisture streaks, wire marks, dirt, roughness, coating uniformity, gloss variation, and misregister in printing.” Col. 1, lns. 7-9. The apparatus includes a strobe “positioned to illuminate a section of the first surface of the paper web during each flash,” and an array camera “positioned adjacent the second surface to receive the transmitted light.” Col. 3, lns. 34-36, 39-40. The visual characteristics are then measured based on “the distribution of transmitted light” through the paper web. Col. 3, lns. 52-53.
U.S. Pat. No. 6,301,373 B1 to Bernie et al. discloses a similar method of determining a quality of sheet material by obtaining “scale of formation information,” where formation is defined as “the local nonuniformity of sheet structure.” Col. 6, lns. 41-45. The scale of formation information is determined using a “visible light transmission technique,” which involves acquiring an image from a sheet placed on a lightbox. Col. 6, lns 3-5.
The referenced patents all concern measuring paper characteristics through transmitted, not reflected, light. Further, the patents concern the detection of formation, not out-of-plane, defects.
U.S. Pat. No. 4,878,114 to Huynh et al. discloses a “method for assessing the roughness of planar surfaces of manufactured products.” Col. 2, lns. 6-8. The disclosed method may also be used to detect the roughness of paper surfaces and to detect surface flaws. Col. 4, lns. 28-59; Col. 5, lns. 10-17. In the disclosed invention, “[a]n area of the surface whose roughness is to be assessed is illuminated by a light source, and the reflected light is directed to the lens of a video camera. The analog output of the video camera is digitized, and the digital signal is provided to a processor which performs an analysis to provide a parameter indicative of the roughness of the surface.” Col. 2, lns. 13-19. In the preferred embodiment, the reflected light is directed to the lens through microscope optics. Col. 2, lns. 67-68. The analysis performed includes obtaining “a histogram of the frequency distribution of the grey levels of the digitized image” and determining a “roughness parameter R” equal to the standard deviation divided by the root-mean-square of the measured grey-levels. Col. 3, lns. 32-45. Further, surface flaws may be detected because they cause the “skewness of the histogram to decrease from a slightly positive value to a negative value.” Col. 4, lns. 46-48. Thus, “one can check the presence of the surface flaws by simply detecting the sign of the skewness of the distribution.” Col. 4, lns. 46-50.
As described above, Huynh et al. concerns the measurement of the roughness of a planar surface, a microscopic characteristic of material related to the average levelness of a surface. Further, the “surface flaw” analysis disclosed by Huynh et al. is used only to detect, not measure, surface flaws. Moreover, surface flaws are detected using a “skewness” value that is a function of both surface flaws and surface roughness. Thus, the analysis disclosed in Huynh et al. does not measure surface flaws independent of other surface characteristics.
Due to the limitations of the aforementioned technologies, the out-of-plane defects of paper ordinarily have been measured subjectively by individual inspection of paper samples. Although these subjective determinations are useful, they are inherently inconsistent and imprecise.